Part 22

The =basi-cornual articulation= is a diarthrosis formed by the junction of each small cornu (kerato-hyal) with the body (basi-hyal). The small cornu has a concave facet which articulates with the convex facet on either end of the dorsal surface of the body. The capsule is ample enough to allow considerable movement, which is chiefly hinge-like. The movements of the hyoid bone are concerned chiefly in the acts of mastication and swallowing. In the latter the distal parts of the hyoid bone are moved forward and upward, carrying the root of the tongue and the larynx with them, and then return to their former position.

THE ARTICULATIONS OF THE THORACIC LIMB

In the absence of the clavicle the thoracic limb forms no articulation with the trunk, unless we regard as such the union by muscles. The movement of the shoulder on the chest-wall is chiefly rotation around a transverse axis passing through the scapula behind the upper part of the spine.

THE SHOULDER JOINT

The shoulder or scapulo-humeral joint (Articulatio scapulo-humeralis) is formed by the junction of the distal end of the scapula with the proximal end of the humerus. The =articular surfaces= are: (1) On the scapula, the glenoid cavity; (2) on the humerus, the head. Both surfaces are approximately spherical and similar in curvature, but the humeral surface is much more extensive than that of the scapula.

The =joint capsule= is ample enough to allow the bones to be drawn apart about an inch (ca. 2 to 3 cm.); but this requires a very considerable amount of force unless air is admitted into the joint cavity. The fibrous layer (or capsular ligament) is not attached to the margin of the joint surfaces, but at a distance of one to two centimeters from it. It is strengthened in front by two diverging elastic bands, which arise on the scapular tuberosity and end on the lips of the bicipital groove. A pad of fat is interposed between the capsule and the tendon of the biceps.

Ligaments are absent from this joint, but the muscles and tendons around it afford remarkable security, so that dislocation seldom or never occurs. The large extent of the head of the humerus is also of importance in this regard.

The principal muscles which are attached around the joint and act as ligaments are: externally, the supraspinatus, infraspinatus, and teres minor; internally, the subscapularis; in front, the biceps and supraspinatus; behind, the triceps.

=Movements.=—While it is a typical enarthrosis in structure, and capable of the various movements of the ball-and-socket joint, the chief normal movements are flexion and extension. In the position of rest the angle formed between the scapula and humerus posteriorly is about 110° to 120°; in flexion it is reduced to about 80°, and in extension it is increased to about 145°. Adduction and abduction are very restricted, the former being limited chiefly by the infraspinatus, the latter by the subscapularis and the low insertion of the superficial pectoral muscles. Rotation is somewhat freer, but does not exceed 33°, when all the muscles are removed (Franck).

THE ELBOW JOINT

This, the cubital articulation (Articulatio cubiti), is a ginglymus formed between the distal extremity of the humerus and the proximal ends of the bones of the forearm.

The =articular surfaces= are: (1) A trochlear surface formed by the condyles of the humerus and the groove between them; (2) the corresponding glenoid cavities and ridge on the proximal extremity of the radius, together with the semilunar notch of the ulna.

The articular surface of the condyles does not extend upon the back of the extremity, but the groove which receives the semilunar notch of the ulna extends up into the olecranon fossa. In the fore part of the groove there is a synovial fossa. The surface on the outer condyle is smaller than that of the inner one, and is subdivided into two unequal parts by a shallow furrow. On the lower part of the semilunar notch and the adjacent part of the ridge on the radius are synovial fossæ.

The =joint capsule= is extremely thin behind, where it forms a pouch in the olecranon fossa under the anconeus muscle and a pad of fat. In front it is strengthened by oblique fibers (Ligamentum obliquum or anterior ligament), and laterally it fuses with the lateral ligaments. Small pouches of the =synovial membrane= lubricate the origins of the flexors of the carpus and digit and the small radio-ulnar joints. There are two lateral ligaments.

The =internal lateral ligament= (Ligamentum collaterale radiale) is attached above to an eminence on the internal epicondyle of the humerus, and divides into two parts: the long, superficial part ends on the inner border of the radius, just below the level of the interosseous space; the deep, short part is inserted into the internal tuberosity of the radius. (The superficial part represents the pronator teres muscle, which is only exceptionally present in the horse.)

The =external lateral ligament= (Ligamentum collaterale ulnare) is short and strong. It is attached above to a depression on the external epicondyle of the humerus, and below to the external tuberosity of the radius, just below the margin of the articular surface.

=Movements.=—This joint is a typical ginglymus, the only movements being flexion and extension around an axis which passes through the upper attachments of the lateral ligaments. In the standing position the articular angle (in front) is about 140° to 150°. The range of movement is about 55° to 60°. Complete extension is prevented chiefly by the tension of the lateral ligaments and the biceps muscle. (The axis of movement is slightly oblique, so that in flexion the forearm is carried somewhat outward.)

THE RADIO-ULNAR ARTICULATION

In the foal the shaft of the ulna is attached to the radius above and below the interosseous space by the =interosseous ligament=. Below the space the two bones become fused before adult age is reached. Above the space the ligament usually persists, but may undergo more or less ossification in extreme old age. The =transverse= or =arciform ligaments= (Ligamentum transversum ulnare et radiale ulnæ et radii) consist of fibers which pass above the interosseous space from either border of the shaft of the ulna to the posterior surface of the radius. The proximal radio-ulnar articulation, formed by two small convex facets on the ulna and the corresponding facets on the posterior surface of the proximal extremity of the radius, is inclosed in the capsule of the elbow joint and does not require separate consideration. The distal extremity of the ulna fuses early with the radius, and is, therefore, regarded usually as a part of the latter.

=Movement.=—This is inappreciable, the forearm being fixed in the position of pronation.

THE CARPAL JOINTS

These joints taken together constitute the composite =articulatio carpi=, or what is popularly termed the “knee-joint” in animals.[33] This consists of three chief joints, viz., (1) The =radio-carpal joint=, formed by the distal end of the radius and the proximal row of the carpus; (2) the =intercarpal joint=, formed between the two rows of the carpus; (3) the =carpo-metacarpal joint=, formed between the distal row of the carpus and the proximal ends of the metacarpal bones. The proximal and middle joints may be regarded as ginglymi, although they are not typical or pure examples of hinge-joints. The distal joint is arthrodial. In addition there are arthrodial joints formed between adjacent bones of the same row (Articulationes interosseæ). All these constitute a very composite joint, with numerous ligaments. The articular surfaces have been described in the Osteology.

The =joint capsule= may be regarded, so far as the fibrous part is concerned, as being common to all three joints. It is attached close to the margin of the articular surface of the radius above and the metacarpus below; its deep face is also attached to a considerable extent to the carpal bones and to the small ligaments. Its anterior part, the =dorsal= or =anterior common ligament=, is rather loose, and assists in forming the fibrous canals for the extensor tendons. Its posterior part, the =volar= or =posterior common ligament=, is very thick and dense, and is closely attached to the carpal bones. It levels up the irregularities of the skeleton here, and forms the smooth anterior wall of the carpal canal. It is continued downward to form the =subcarpal= or =inferior check ligament=, which blends with the tendon of the flexor perforans about the middle of the metacarpus.

The =synovial membrane= forms three sacs corresponding to the three joints. The =radio-carpal sac= is the most voluminous; it includes the joints formed by the accessory carpal bone, and also those between the proximal carpal bones as far as the interosseous ligaments. The =intercarpal sac= sends extensions upward and downward between the bones of the two rows as far as the interosseous ligaments; it communicates between the third and fourth carpal bones with the =carpo-metacarpal sac=. The latter is very limited in extent, and is closely applied to the bones; it incloses the carpo-metacarpal joint, and lubricates also the lower parts of the joints between the distal carpal bones and the intermetacarpal joints.

The =external lateral ligament= (Ligamentum carpi collaterale ulnare) is attached above to the external tuberosity of the distal end of the radius. Its long superficial part is attached below to the proximal end of the external small metacarpal chiefly, but some fibers end on the large metacarpal bone. A canal for the lateral extensor tendon separates a short deep band which ends on the ulnar carpal bone. Other deep fibers connect the latter with the fourth carpal bone, and the fourth carpal with the metacarpus.

The =internal lateral ligament= (Ligamentum carpi collaterale radiale) resembles the preceding in general, but is stronger and wider distally. It is attached above to the internal tuberosity of the distal end of the radius and ends below on the proximal ends of the large and inner small metacarpal bones. Deep fasciculi are detached to the radial and second carpal bones. The first carpal bone, when present, is usually embedded in the posterior part of the distal end of the ligament. The posterior part of the ligament is fused with the posterior annular ligament (Ligamentum carpi transversum), and concurs in the formation of a canal for the tendon of the flexor carpi internus.

A number of special short ligaments connect two or more adjacent bones; only the most distinct of these will be described here.

The accessory carpal bone is connected with adjacent bones by three ligaments (Fig. 444). The proximal one is a short band which extends from the accessory carpal in front of the groove on its outer face and is inserted into the distal end of the radius behind the groove for the lateral extensor tendon. A middle band connects the accessory with the ulnar carpal. The distal ligament consists of two strong bands which pass from the lower margin of the accessory to the fourth carpal and the proximal end of the outer metacarpal bone; these bands transmit the action of the muscles, which are inserted into the accessory carpal bone. The other bones of the proximal row are connected by two anterior or dorsal ligaments, which are transverse in direction, and two interosseous ligaments. An oblique ligament passes from an eminence on the posterior surface of the radial carpal bone to a small depression on the radius internal to the facet for the accessory carpal bone.

Two ligaments connect the proximal and distal rows posteriorly. The inner one joins the radial to the second and third carpal, and the outer one attaches the ulnar to the third and fourth carpals.

The bones of the distal row are connected by two strong transverse anterior or dorsal ligaments and two interosseous ligaments.

There are four carpo-metacarpal ligaments. Two oblique anterior bands connect the third carpal with the large metacarpal. Two interosseous ligaments pass downward from the interosseous ligaments of the distal row to end in depressions in the interstices between the proximal ends of the metacarpal bones.

=Movements.=—Taking the joint as a whole, the chief movements are flexion and extension. In the standing position the joint is extended. When the joint is flexed, slight lateral movement and rotation can be produced by manipulation. The anterior part of the capsule is, of course, tense during flexion, the posterior part in extension.

The movement practically all occurs at the radio-carpal and intercarpal joints, the articular surfaces of which are widely separated in front during flexion, but remain in contact behind. The distal row remains in contact with the metacarpus. The intermediate and ulnar carpals move together as one piece, but the radial does not move so far as the intermediate, so that the anterior and interosseous ligaments connecting these bones become tense and oblique in direction.

THE FETLOCK JOINT

This, the =metacarpo-phalangeal articulation= (Articulatio metacarpo-phalangea), is a ginglymus formed by the junction of the distal end of the large (third) metacarpal bone, the proximal end of the first phalanx, and the proximal sesamoid bones.

=Articular Surfaces.=—The surface on the large metacarpal bone is approximately cylindrical in curvature, but is divided into two slightly unequal parts by a sagittal ridge. This is received into a sort of socket formed by the first phalanx below and the two sesamoids together with the intersesamoid ligament behind. The latter is a mass of fibro-cartilage in which the sesamoid bones are largely embedded. It extends above the level of the sesamoids, and is grooved to receive the ridge on the metacarpal bone; its posterior surface forms a smooth groove for the deep flexor tendon.

The =joint capsule= is attached around the margin of the articular surfaces. It is thick and ample in front; here a bursa is interposed between it and the extensor tendons, but the tendons are also attached to the capsule. Posteriorly it forms a thin-walled pouch which extends upward between the metacarpal bone and the suspensory ligament about as high as the point of bifurcation of the latter. The capsule is reinforced by two lateral ligaments.

The =lateral ligaments=, =external= and =internal= (Ligamentum collaterale ulnare, radiale) are partially divided into two layers: the =superficial layer= arises from the eminence on the side of the distal end of the large metacarpal bone, and passes straight to the rough lateral area below the margin of the articular surface of the first phalanx; the =deep layer=, shorter and much stronger, arises in the lateral depression on the distal end of the metacarpal bone, and passes obliquely downward and backward to be inserted into the outer surface of the sesamoid and the proximal end of the first phalanx.

The capsule is further strengthened by a layer of oblique fibers which pass over the lateral ligament on either side and end on the extensor tendon and the proximal extremity of the first phalanx. It may properly be regarded as fascia rather than ligament.

=Movements.=—These are of the nature of flexion and extension, the axis of motion passing through the upper attachments of the lateral ligaments. In the ordinary standing position the joint is in a state of partial =dorsal flexion=, the articular angle (in front) being about 140° to 150°. (In the hind limb it is about 5° greater.) Diminution of this angle (sometimes termed “overextension”) is normally very limited on account of the resistance offered by the sesamoidean apparatus, but it varies considerably in amount in different subjects. =Volar flexion= is limited only by contact of the heels with the metacarpus. During volar flexion a small amount of lateral flexion is possible.

THE SESAMOIDEAN LIGAMENTS

Under this head will be described a number of important ligaments which are connected with the sesamoid bones and form a sort of stay apparatus or brace.

The =intersesamoidean ligament= (Ligamentum intersesamoideum) not only fills the space between and unites the sesamoid bones, but also extends above them, entering into the formation of the articular surface of the fetlock joint. Other facts in regard to it have been given above.

The =lateral sesamoidean ligaments=, outer and inner (Ligamenta sesamoidea ulnare et radiale), arise on the abaxial surface of each sesamoid bone, pass forward, and divide into two branches, one of which ends in the depression on the distal end of the large metacarpal bone, the other on the eminence on the proximal end of the first phalanx. They are partly covered by the branches of the suspensory or superior sesamoidean ligament.

The =suspensory= or =superior sesamoidean ligament= (Musculus interosseus medius) lies for the greater part in the metacarpal groove, where it has the form of a wide, thick band. It is attached above to the upper part of the posterior surface of the large metacarpal bone and to the distal row of carpal bones. At the lower fourth of the metacarpus it divides into two diverging branches. Each branch passes to the abaxial face of the corresponding sesamoid, on which a considerable part, is attached. The remainder passes obliquely downward and forward to the anterior surface of the first phalanx, where it joins the extensor tendon. This ligament possesses considerable elasticity, and is the highly modified interosseous medius muscle. It consists mainly of tendinous tissue, but contains a variable amount of striped muscular tissue, especially in its deep part and in young subjects. Its principal function is to support the fetlock, _i. e._, to prevent excessive dorsal flexion of the joint when the weight is put on the limb. The branches which join the common extensor tendon limit volar flexion of the interphalangeal joints in certain phases of movement.

The =inferior sesamoidean ligaments= are three in number—superficial, middle, and deep. The =superficial= or =straight ligament= (Ligamentum sesamoideum rectum) is a flat band and is somewhat wider above than below.[34] It is attached above to the bases of the sesamoid bones and the intersesamoid ligament, below to the complementary fibro-cartilage of the proximal end of the second phalanx. The =middle ligament= is triangular, with thick, rounded margins (Ligamenta obliqua) and a thin central portion.[35] Its base is attached to the sesamoid bones and intersesamoid ligament, and its deep face to the triangular rough area on the posterior surface of the first phalanx. The =deep= or =cruciate ligament= (Ligamenta sesamoidea cruciata) consists of two thin layers of fibers which arise on the base of the sesamoid bones, cross each other, and end on the opposite eminence on the proximal end of the first phalanx.

The =short sesamoidean ligaments= (Ligamenta sesamoidea brevia) are best seen by opening the joint in front and pushing the sesamoid bones backward; they are covered by the synovial membrane. Each is a short band which extends from the anterior part of the base of the sesamoid bone outward to the posterior margin of the articular surface of the first phalanx.

The inferior sesamoidean ligaments may be regarded as digital continuations of the suspensory ligament, the sesamoid bones being intercalated in this remarkable stay apparatus by which the fetlock is supported and concussion diminished.

THE PASTERN JOINT

This, the =proximal interphalangeal articulation= (Articulatio phalangis secundæ), is a ginglymus formed by the junction of the distal end of the first phalanx and the proximal end of the second phalanx.

The =articular surfaces= are: (1) On the first phalanx, two slightly unequal convex areas with an intermediate shallow groove; (2) on the second phalanx, a corresponding surface, completed behind by a plate of fibro-cartilage.

The =joint capsule= is close fitting in front and laterally, where it blends with the extensor tendon and the lateral ligaments respectively. Behind it pouches upward a little and is reinforced by the straight sesamoidean ligament and the branches of the superficial flexor tendon.

There are two lateral and four volar ligaments.

The =lateral ligaments=, internal and external (Ligamentum collaterale radiale, ulnare) are very short and strong bands which are attached above on the eminence and depression on each side of the distal end of the first phalanx, and below on the eminence on either side of the proximal end of the second phalanx. The direction of the ligaments is about vertical and, therefore, does not correspond to the digital axis.

The =volar= or =posterior ligaments= consist of central and lateral pairs of bands which are attached below to the posterior margin of the proximal end of the second phalanx and its complementary fibro-cartilage. The lateral pair is attached above to the middle of the borders of the first phalanx, the central pair lower down and on the margin of the triangular rough area.